4 Answers
4

Runtime Broker process was created by Microsoft and is a core process in Windows 8.

RuntimeBroker.exe is a safe Microsoft process included in Windows 8 to assist with Metro app permissions. With a light system footprint of more or less 5,000 K of RAM being used, it does not affect performance.

RuntimeBroker.exe is triggered by Metro apps. If there are no Metro apps open, this process won't run.

What does it do?

The Runtime Broker is responsible for checking if a Metro app is declaring all of its permissions (like accessing your Photos) and informing the user whether or not its being allowed. In particular it is interesting to see how it functions when paired with access to hardware, such as an app’s ability to take webcam snapshots. It's serves as a middleman between your apps and your privacy/security.

A quick look through the strings of the process shows the Microsoft definition of Runtimebroker.exe to be part of “Processes for Windows Partial Trust Components.”
Most of its related registry entries and the process itself can be found at:

RuntimeBroker.exe is as described above by amiregelz; however it can also be a memory hog that can slow Windows 8 to a crawl. There are apparently situations where Windows Store Apps (a.k.a. Metro Apps) cause/have a memory leak when scheduling live tile updates. I recently discovered that the RuntimeBroker was using ~2.5GB of memory -- which of course caused some perf problems with my system.
See http://social.technet.microsoft.com/Forums/en-AU/w8itprogeneral/thread/52000c8f-f2b7-4c10-a4ec-01adfcdf2f39 for a discussion and more details on the problem.

Obviously there have been users who've experienced memory related issues with this particular .exe - but I'm not one, so I can't attest to any memory issues.

However, I have noticed this under active processes and it is almost always there after the system has been idle for a while.

I believe RuntimeBroker.exe becomes active after an idle period, where it functions as a relay, communicating with any apps that remained opened during the idle period - checking to see if they should be closed or become active once again.

*I do not work with computers, I'm only an average user. This is only my opinion and shouldn't be used as the basis for making any changes to any machine.